1. The Field of the Invention
The present invention relates to semiconductor processing and more particularly to a process for forming openings in layered structures such as tungsten silicide (WSi) over polycrystalline silicon (polysilicon).
2. The Relevant Art
Semiconductor processing to create integrated circuits and the like involves repeated cycles through a series of masking operations, processing operations, and cleaning operations. Some passes through the cycle create new layers, some form holes in existing layers, others remove layers, and still others modify exposed surfaces. FIG. 1 illustrates a cross-section of a typical semiconductor stack 10 of the prior art that is commonly fabricated in the manufacture of integrated circuits. The stack 10 includes a substrate 12, a dielectric layer 14, a polysilicon layer 16, a WSi layer 18, and a photomask 20. An opening 22 is formed over a desired location on the dielectric layer 14 by first forming an opening in the photomask 20 by photolithography techniques that are well known in the art. The opening in the photomask 20 allows the exposed surface to be further processed, in this case to form an opening in the WSi layer 18 and the polysilicon layer 16. Many different techniques are known in the art for etching semiconductor layers, but for etching WSi 18 over polysilicon 16 a frequently used technique involves plasma etching with a mixture of chlorine gas and oxygen gas.
Many semiconductor devices presently manufactured include numerous semiconductor stacks 10 in successive layers. The fabrication of such devices is made more complex by the topography that develops after several stacks 10 have already been created. FIG. 2 shows a cross-section of a more complex stack 24 during the formation of openings 22 in two separate locations. Stack 24 includes a polysilicon layer 26 with a topography that is conformal with the layer beneath it (not shown). Typically, this underlying layer is a dielectric layer such as a thermally grown silicon dioxide film that itself has a topography dictated by the layers beneath it. Formed over the polysilicon layer 26 is a WSi layer 28. WSi layer 28 is commonly a thick layer that can be mechanically or chemically polished to form a flat surface 29, as shown in FIG. 2. Stack 24 also includes a photomask 20 disposed over the WSi layer 28. The photomask 20 further includes openings 22 over portions of the WSi layer 28 having different thicknesses.
In FIG. 2, a plasma etch with a mixture of chlorine and oxygen gases is applied to the stack 24 to etch the WSi layer 28 where exposed by the openings 22 in the photomask 20. This etch is referred to as a silicide etch. Openings 22 grow deeper at approximately the same rate until one advances to the polysilicon layer 26 as illustrated. At this point, because polysilicon generally etches at a faster rate than WSi when etched with a plasma of chlorine and oxygen, the first opening to reach the polysilicon layer 26 will etch through to the dielectric layer 14 not only well before the other opening, but possibly before the other opening even reaches the polysilicon layer 26. Commonly, a silicide overetch is applied to remove any residual silicide remaining in the openings 22 at the completion of the initial silicide etch. The silicide overetch is typically nothing more than the initial silicide etch applied for an additional length of time.
Selectivity is a term commonly used in the art to represent the ratio between the etch rates of two different materials under common conditions. The ratio of the WSi etch rate to the polysilicon etch rate, commonly known as the polycide selectivity, is well known to be less than one for plasma etching with most ratios of chlorine to oxygen. The lower the polycide selectivity the greater the disparity between the depths of the several openings 22 where the thickness of the WSi layer 28 is not uniform.
Following the silicide overetch a second plasma etch using a mixture of hydrogen bromide (HBr) and chlorine gases, commonly referred to as a poly etch, is applied. The poly etch etches polysilicon well but etches WSi poorly. Thus, the poly etch has a low selectivity for WSi over polysilicon, necessitating the preceding silicide overetch to make sure residual WSi is removed as the later processing is not likely to remove it. Lastly, a poly overetch is applied to clean up any exposed residual portions of the polysilicon layer 26. A poly overetch should have a high selectivity for polysilicon over the dielectric. Put another way, the poly overetch ideally should etch polysilicon but not silicon dioxide so that the ratio of their etch rates approaches infinity. A common poly overetch employs a plasma of HBr, He, and O2.
It will be apparent, therefore, that the prior art calls for four etching steps to form contact openings in a polycide layer where the layer thicknesses are not everywhere uniform, a first etch to substantially remove silicide, a second etch to complete the silicide removal, a third etch to substantially remove polysilicon, and a fourth etch to complete the polysilicon removal.
In addition to mixtures of chlorine and oxygen, other gas mixtures have been attempted. U.S. Pat. No. 5,914,276 to Shin et al. discloses using a mixture of chlorine and nitrogen gases at preferred volumetric flow rates where the nitrogen is 5% to 20% of the total. This range of flow rates was found to strike the proper balance between protecting the silicide/poly interface from lateral overetching and complete removal of the desired portions. A volumetric flow rate for nitrogen above 20% of the total was found to lead to difficulties controlling the critical dimension of the pattern where the metal silicide layer was titanium silicide. Shin, however, was not concerned with the problem of complex topographies where the silicide layer thickness is not everywhere the same at the start of the etch. Therefore, achieving polycide selectivities greater than one was not a goal of Shin.
What is desired, therefore, is a new etch with a polycide selectivity greater than one that is effective to replace the silicide overetch and the poly etch of the prior art and therefore simplify the formation of contact openings in a polycide layer that includes a silicide layer with a varying thickness.
The present invention discloses a method for etching a plurality of contact openings in a polycide layer where the result of underlying topography creates a silicide layer with a varying thickness. The method of the present invention includes providing a polycide layer disposed over a substrate, forming a patterned mask layer over the polycide layer, and selectively etching exposed portions of the polycide layer with a series of three different etches to form a plurality of contact openings that each expose the substrate. In some embodiments a dielectric layer is disposed between the substrate and the polycide layer and is exposed by the contact openings instead of the substrate. The polycide layer of the present invention includes a polysilicon layer disposed above the substrate and a metal silicide layer disposed above the polysilicon layer. In some embodiments the metal silicide is tungsten silicide. Processes for the formation of the various layers and the patterning of the photoresist layer are well known in the art.
According to the method of the invention the three etches that are applied to form the plurality of contact openings are a silicide etch, a polycide etch, and a poly overetch. The silicide etch defines a plurality of contact openings in the silicide layer where exposed by the patterned photoresist layer by substantially removing the exposed portions of the silicide layer. The silicide etch in some embodiments comprises a plasma of CF4, Cl2, and N2.
The polycide etch extends the plurality of contact openings towards the substrate and continues until the substrate has been exposed by at least one of the plurality of contact openings. The polycide etch employs a plasma of N2 and Cl2 where the N2 is supplied at a flow rate between 20% and about 30%. Mixtures of chlorine and nitrogen in this range advantageously have polycide selectivities greater than one. A selectivity greater than one implies that a contact opening that has advanced into the polysilicon layer will etch more slowly than one still advancing through the silicide layer.
The poly overetch has a high selectivity for polysilicon compared to the substrate. Consequently, the poly overetch has little effect on exposed portions of the substrate but continues to remove polysilicon from those contact openings that are not fully defined. In those embodiments that include a dielectric layer the poly overetch has instead a high selectivity for polysilicon compared to the dielectric layer. The poly overetch is continued until each of the plurality of contact openings exposes the substrate or the dielectric layer. The poly overetch in some embodiments comprises a plasma of HBr, He, and O2, and in further embodiments further includes N2. These
These and other aspects and advantages of the present invention will become more apparent when the description below is read in conjunction with the accompanying drawings.